An Evidence Basis for Future Equestrian Helmet Lateral Crush Certification Tests

Abstract

The aim of this study is to determine what loads are likely to be applied to the head in the event of a horse falling onto it and to determine by how much a typical equestrian helmet reduces these loads. An instrumented headform was designed and built to measure applied dynamic loads from a falling horse. Two differently weighted equine cadavers were then dropped repeatedly from a height of 1 m (theoretical impact velocity of 4.43 m/s) onto both the un-helmeted and helmeted instrumented headforms to collect primary force–time history data. The highest mean peak loads applied to the headform by the lighter horse were measured at the bony sacral impact location (15.57 kN ± 1.11 SD). The lowest mean peak loads were measured at the relatively fleshier right hind quarter (7.91 kN ± 1.84 SD). For the heavier horse, highest mean peak loads applied to the headform were measured at the same bony sacral impact location (16.02 kN ± 0.83 SD), whilst lowest mean peak loads were measured at the more compliant left hind quarter (10.47 kN ± 1.08 SD). When compared with the un-helmeted mean values, a reduction of 29.7% was recorded for the sacral impact location and a reduction of 43.3% for the lumbosacral junction location for helmeted tests. Notably, all measured loads were within or exceeded the range of published data for the fracture of the adult lateral skull bone. Current helmet certification tests are not biofidelic and inadequately represent the loading conditions of real-world “lateral crush” accidents sustained in equestrian sports. This work presents the first ever evidence basis upon which any future changes to a certification standards test method might be established, thereby ensuring that such a test would be both useful, biofidelic, and could ensure the desired safety outcome.

Full text

applied
sciences
Article
An Evidence Basis for Future Equestrian Helmet
Lateral Crush Certification Tests
Thomas A. Connor 1,2,3, J. Michio Clark 1,4, Pieter Brama 5, Matt Stewart 2,
Aisling NíAnnaidh 1and Michael D. Gilchrist 1, *
1School of Mechanical & Materials Engineering, University College Dublin, Belfield, 4 Dublin, Ireland;
thomas.connor@ucdconnect.ie (T.A.C.); mclark@vectorscientific.com (J.M.C.);
aisling.niannaidh@ucd.ie (A.N.A.)
2COMFG Ltd. (Charles Owen), Royal Works, Croesfoel Ind. Park, Wrexham LL14 4BJ, UK;
mattstewart@charlesowen.co.uk
3R&D Consulting Engineers Ltd., Leeds LS17 6AF, UK
4Vector Scientific Inc., Golden, CO 80403, USA
5School of Veterinary Medicine, University College Dublin, Belfield, 4 Dublin, Ireland; pieter.brama@ucd.ie
*Correspondence: michael.gilchrist@ucd.ie
Received: 20 March 2020; Accepted: 3 April 2020; Published: 10 April 2020


Abstract:
The aim of this study is to determine what loads are likely to be applied to the head in the
event of a horse falling onto it and to determine by how much a typical equestrian helmet reduces
these loads. An instrumented headform was designed and built to measure applied dynamic loads
from a falling horse. Two differently weighted equine cadavers were then dropped repeatedly from
a height of 1 m (theoretical impact velocity of 4.43 m/s) onto both the un-helmeted and helmeted
instrumented headforms to collect primary force–time history data. The highest mean peak loads
applied to the headform by the lighter horse were measured at the bony sacral impact location
(
15.57 kN ±1.11 SD
). The lowest mean peak loads were measured at the relatively fleshier right
hind quarter (7.91 kN
±
1.84 SD). For the heavier horse, highest mean peak loads applied to the
headform were measured at the same bony sacral impact location (16.02 kN
±
0.83 SD), whilst lowest
mean peak loads were measured at the more compliant left hind quarter (10.47 kN
±
1.08 SD). When
compared with the un-helmeted mean values, a reduction of 29.7% was recorded for the sacral impact
location and a reduction of 43.3% for the lumbosacral junction location for helmeted tests. Notably,
all measured loads were within or exceeded the range of published data for the fracture of the adult
lateral skull bone. Current helmet certification tests are not biofidelic and inadequately represent the
loading conditions of real-world “lateral crush” accidents sustained in equestrian sports. This work
presents the first ever evidence basis upon which any future changes to a certification standards test
method might be established, thereby ensuring that such a test would be both useful, biofidelic, and
could ensure the desired safety outcome.
Keywords: skull fracture; dynamic crush; lateral crush; roll over; head protection
1. Introduction
Equestrian helmet certification tests are designed to ensurethat a minimum performance and quality
level is achieved in terms of helmet crashworthiness and structural integrity. As equestrian sports are high
risk [
1
–
5
], with the primary type of accident involving a fall from the horse resulting in a head impact [
6
],
it makes good sense that the main helmet functional test in the standards involves recreating some
simplified impact conditions [
7
–
9
]. The next most significant test in most equestrian helmet standards is
referred to as the lateral crush test, also referred to as the lateral deformation test or rigidity test. However,
Appl. Sci. 2020,10, 2623; doi:10.3390/app10072623 www.mdpi.com/journal/applsci

Appl. Sci. 2020,10, 2623 2 of 11
unlike impact tests, the origins of which are well documented in the literature [
10
,
11
], the rationale and
evidence basis for the crush test are unclear. Essentially, this particular test is formulated as a quasi-static
test to represent a horse dynamically falling against or rolling over the head of a helmeted jockey.
The lateral crush test itself is relatively simple. A helmet is placed between two metal plates and is
crushed quasi-statically until a peak force is reached at a specified loading rate (see Figure 1). There is
no headform in the helmet. To pass the test, maximum and residual crush limits must not be exceeded.
Peak loads are set to be 800 N for both PAS 015 [
9
] and EN 1384 [
8
] and 1000 N for the Snell E2016
standard [
12
]. In all cases, the maximum permitted crush is 30 mm and the residual crush may not
exceed 10 mm.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 2 of 12
deformation test or rigidity test. However, unlike impact tests, the origins of which are well
documented in the literature [10,11], the rationale and evidence basis for the crush test are unclear.
Essentially, this particular test is formulated as a quasi-static test to represent a horse dynamically
falling against or rolling over the head of a helmeted jockey.
The lateral crush test itself is relatively simple. A helmet is placed between two metal plates and
is crushed quasi-statically until a peak force is reached at a specified loading rate (see Figure 1). There
is no headform in the helmet. To pass the test, maximum and residual crush limits must not be
exceeded. Peak loads are set to be 800 N for both PAS 015 [9] and EN 1384 [8] and 1000 N for the Snell
E2016 standard [12]. In all cases, the maximum permitted crush is 30 mm and the residual crush may
not exceed 10 mm.
Figure 1. Lateral crush test.
In discussions with engineers working within the standards industry and with standards
committee members, it is understood that the lateral crush tests are used to ensure that the helmet is
‘not too soft’ and that the structure of the helmet has some ‘stabilizing effect’. It is not intended to
simulate a real-world accident. However, there has been no quantification of what constitutes a
helmet that is ‘too soft’, particularly if its impact performance is sufficient. Additionally, in
discussions with the equestrian community, it is clear that the lateral crush test is believed to
represent a horse falling onto a helmet. Indeed, the most recent revision of the EN1384 standard was
to increase the peak force that could be sustained from 630 N to 800 N. That decision was taken on
the basis that it should improve helmet performance in the event of a horse falling onto a rider’s head.
However, there is no evidence that this change would have any influence on helmet performance.
Equestrians have a high risk of head injury [13] and the majority of professional jockey fatalities
are as a result of head injury sustained from a fall. Additionally, reported rates of concussion or mild
traumatic brain injury (mTBI) are higher for equestrians than those in boxing and American football
[6]. However, when compared with the number of falls and head impacts, crush injuries, particularly
to the head, appear to be rare with few reported in the literature [14]. Nevertheless, in some situations
such as cross-country riding or eventing, a horse can somersault during a jump and land on the rider.
In such cases the injuries can be catastrophic and sometimes fatal [14]. There may be merit in
introducing a more realistic crush test to the standards if the objective of the test is to improve helmet
performance while being dynamically crushed. However, there are no primary empirical data on
which to base such a test. It is not known what typical loads are applied to the rider’s head during
such an accident and it is not known by how much a typical equestrian helmet might reduce these
loads.
The aim of this present paper is to address this deficit directly by determining what loads are
likely to be applied to the rider’s head in the event of a horse falling onto it, and to investigate the
Figure 1. Lateral crush test.
In discussions with engineers working within the standards industry and with standards committee
members, it is understood that the lateral crush tests are used to ensure that the helmet is ‘not too
soft’ and that the structure of the helmet has some ‘stabilizing effect’. It is not intended to simulate
a real-world accident. However, there has been no quantification of what constitutes a helmet that is ‘too
soft’, particularly if its impact performance is sufficient. Additionally, in discussions with the equestrian
community, it is clear that the lateral crush test is believed to represent a horse falling onto a helmet.
Indeed, the most recent revision of the EN1384 standard was to increase the peak force that could be
sustained from 630 N to 800 N. That decision was taken on the basis that it should improve helmet
performance in the event of a horse falling onto a rider’s head. However, there is no evidence that this
change would have any influence on helmet performance.
Equestrians have a high risk of head injury [
13
] and the majority of professional jockey fatalities
are as a result of head injury sustained from a fall. Additionally, reported rates of concussion or mild
traumatic brain injury (mTBI) are higher for equestrians than those in boxing and American football [
6
].
However, when compared with the number of falls and head impacts, crush injuries, particularly to the
head, appear to be rare with few reported in the literature [
14
]. Nevertheless, in some situations such as
cross-country riding or eventing, a horse can somersault during a jump and land on the rider. In such
cases the injuries can be catastrophic and sometimes fatal [
14
]. There may be merit in introducing a more
realistic crush test to the standards if the objective of the test is to improve helmet performance while
being dynamically crushed. However, there are no primary empirical data on which to base such a test.
It is not known what typical loads are applied to the rider’s head during such an accident and it is not
known by how much a typical equestrian helmet might reduce these loads.
The aim of this present paper is to address this deficit directly by determining what loads are
likely to be applied to the rider’s head in the event of a horse falling onto it, and to investigate the

Appl. Sci. 2020,10, 2623 3 of 11
extent to which a typical equestrian helmet reduces these loads. It is hoped that these primary data
will help to inform and create an evidence basis for future standard lateral crush tests.
2. Materials and Methods
This study has two main parts. First, an instrumented headform was designed and built to
measure applied dynamic loads from a falling horse. Second, equine cadavers were dropped onto
both un-helmeted and helmeted instrumented headforms and the associated force–time history data
were collected.
2.1. Instrumented Headform
To measure the lateral forces applied by the falling horse, the external geometry of the EN960:2006
standard headform was used to create a CAD model (see Figure 2). The headform was modelled on
size J (the average male). A Makerbot Z18 3D printer was used to physically print the headform from
this CAD model, similar to [
15
]. To make the printed poly-lactic-acid (PLA) size J headform, Monsterfil
1.75 mm PLA filament was printed at 100% density in 0.2 mm layers. The print had a two-shell outer
surface and a linear infill. The headform was fitted with a Kistler uniaxial load cell rated to 70 kN and
data acquisition was by means of a Kistler single channel laboratory amplifier. Data were filtered to
ISO 6487 [16].
Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 12
extent to which a typical equestrian helmet reduces these loads. It is hoped that these primary data
will help to inform and create an evidence basis for future standard lateral crush tests.
2. Materials and Methods
This study has two main parts. First, an instrumented headform was designed and built to
measure applied dynamic loads from a falling horse. Second, equine cadavers were dropped onto
both un-helmeted and helmeted instrumented headforms and the associated force–time history data
were collected.
2.1. Instrumented Headform
To measure the lateral forces applied by the falling horse, the external geometry of the
EN960:2006 standard headform was used to create a CAD model (see Figure 2). The headform was
modelled on size J (the average male). A Makerbot Z18 3D printer was used to physically print the
headform from this CAD model, similar to [15]. To make the printed poly-lactic-acid (PLA) size J
headform, Monsterfil 1.75 mm PLA filament was printed at 100% density in 0.2 mm layers. The print
had a two-shell outer surface and a linear infill. The headform was fitted with a Kistler uniaxial load
cell rated to 70 kN and data acquisition was by means of a Kistler single channel laboratory amplifier.
Data were filtered to ISO 6487 [16].
Figure 2. (a) Split headform exploded view. 1) Printed left- and right-hand sides, 2) uni-axel load cell,
3) load distribution plates, 4) mounting bolts. (b) Split headform assembly.
2.2. Equine Cadaver Drop Tests
For the drop tests, two fresh equine cadavers were used, one 343 kg female (horse 1) and one
370 kg male (horse 2). Both animals had been euthanised for reasons unrelated to the present study
on the day of testing. Full ethical exemption was approved (AREC-E-17-09).
The equine cadavers were dropped from a height of 1.2 m onto the instrumented J headform
which was positioned on a rigid concrete surface. The drop height was chosen following analysis of
real-world equestrian accident video footage, such as described by Connor et al., [13] and Clark et al.,
[17]. The concrete surface had been chosen as it was essentially rigid and served to eliminate surface
variability.
A Manatu forklift and lifting beam was used to lift the cadavers to the drop height (see Figure
3). A quick release hook clamp was used to drop the cadaver. For the un-helmeted tests, four impact
locations were chosen on each horse, the left hind quarter, the right hind quarter, lumbosacral
vertebrae, and the sacral vertebrae (see Figure 4). These impact locations were chosen based on the
analysis of video footage of horse falls and they also represented the largest area of the animal that is
not covered by a saddle, apart from the head and neck. For each horse, 3 drops were carried out per
impact location. The helmet model used was a commonly available 57 cm jockey style equestrian
helmet, certified to ASTM F1163-15 [7], EN 1384 [8] and PAS015 [9]. and Ideally, other equestrian
Figure 2.
(
a
) Split headform exploded view. (1) Printed left- and right-hand sides, (2) uni-axel load cell,
(3) load distribution plates, (4) mounting bolts. (b) Split headform assembly.
2.2. Equine Cadaver Drop Tests
For the drop tests, two fresh equine cadavers were used, one 343 kg female (horse 1) and one
370 kg male (horse 2). Both animals had been euthanised for reasons unrelated to the present study on
the day of testing. Full ethical exemption was approved (AREC-E-17-09).
The equine cadavers were dropped from a height of 1.2 m onto the instrumented J headform
which was positioned on a rigid concrete surface. The drop height was chosen following analysis of
real-world equestrian accident video footage, such as described by Connor et al., [
13
] and Clark et al., [
17
].
The concrete surface had been chosen as it was essentially rigid and served to eliminate surface variability.
A Manatu forklift and lifting beam was used to lift the cadavers to the drop height (see Figure 3).
A quick release hook clamp was used to drop the cadaver. For the un-helmeted tests, four impact
locations were chosen on each horse, the left hind quarter, the right hind quarter, lumbosacral vertebrae,
and the sacral vertebrae (see Figure 4). These impact locations were chosen based on the analysis of
video footage of horse falls and they also represented the largest area of the animal that is not covered
by a saddle, apart from the head and neck. For each horse, 3 drops were carried out per impact location.
The helmet model used was a commonly available 57 cm jockey style equestrian helmet, certified to

Appl. Sci. 2020,10, 2623 4 of 11
ASTM F1163-15 [
7
], EN 1384 [
8
] and PAS015 [
9
]. and Ideally, other equestrian helmet models would
also have been tested. However, due to availability and price constraints, this common and widely
used helmet model was chosen as a good representative of commercially available helmets for the 50th
percentile male sized head. Helmeted tests were carried out on the lumbosacral vertebrae junction
and the sacral vertebrae locations, as these were found to be the most stable locations in terms of
drop test repeatability. Additionally, only the heavier male animal was used for the helmeted tests,
as the number of helmets available was limited to six. The force–time data were recorded for each
drop test. In total, 30 drop tests were carried out: 24 un-helmeted tests and 6 helmeted tests. Means
and standard deviations were calculated for repeated tests at each impact location and for all impact
locations combined. Peak load data were analysed statistically using a one-way ANOVA with post-hoc
t-tests (
α
=0.05) between helmeted and un-helmeted tests and between impact locations to determine
statistically significant differences.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 12
helmet models would also have been tested. However, due to availability and price constraints, this
common and widely used helmet model was chosen as a good representative of commercially
available helmets for the 50th percentile male sized head. Helmeted tests were carried out on the
lumbosacral vertebrae junction and the sacral vertebrae locations, as these were found to be the most
stable locations in terms of drop test repeatability. Additionally, only the heavier male animal was
used for the helmeted tests, as the number of helmets available was limited to six. The force–time
data were recorded for each drop test. In total, 30 drop tests were carried out: 24 un-helmeted tests
and 6 helmeted tests. Means and standard deviations were calculated for repeated tests at each
impact location and for all impact locations combined. Peak load data were analysed statistically
using a one-way ANOVA with post-hoc t-tests (α = 0.05) between helmeted and un-helmeted tests
and between impact locations to determine statistically significant differences.
Figure 3. Cadaver horse drop test set up.
Figure 4. (a) Left hind quarter impact location. (b) Sacral vertebrae impact location. (c) Lumbosacral
vertebrae impact location. (d) Right hind quarter impact location.
3. Results
Force–time data were successfully collected for all 30 drop tests with the split headform proving
to be reliable and repeatable.
3.1. Horse 1 Drop Tests
Horse 1 was the lighter of the two horses, weighting 343 kg. The most repeatable data were
collected from the lumbosacral junction and sacral vertebrae impact locations (see Figure 5b,c). Both
left and right hind quarter locations were less reliable as the headform was pushed out of position in
some impacts (see Figure 5a,d). The highest mean peak loads applied to the headform were measured
at the sacral impact location (15.57 kN). The lowest mean peak loads were measured at the right hind
quarter (7.91 kN).
Figure 3. Cadaver horse drop test set up.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 12
helmet models would also have been tested. However, due to availability and price constraints, this
common and widely used helmet model was chosen as a good representative of commercially
available helmets for the 50th percentile male sized head. Helmeted tests were carried out on the
lumbosacral vertebrae junction and the sacral vertebrae locations, as these were found to be the most
stable locations in terms of drop test repeatability. Additionally, only the heavier male animal was
used for the helmeted tests, as the number of helmets available was limited to six. The force–time
data were recorded for each drop test. In total, 30 drop tests were carried out: 24 un-helmeted tests
and 6 helmeted tests. Means and standard deviations were calculated for repeated tests at each
impact location and for all impact locations combined. Peak load data were analysed statistically
using a one-way ANOVA with post-hoc t-tests (α = 0.05) between helmeted and un-helmeted tests
and between impact locations to determine statistically significant differences.
Figure 3. Cadaver horse drop test set up.
Figure 4. (a) Left hind quarter impact location. (b) Sacral vertebrae impact location. (c) Lumbosacral
vertebrae impact location. (d) Right hind quarter impact location.
3. Results
Force–time data were successfully collected for all 30 drop tests with the split headform proving
to be reliable and repeatable.
3.1. Horse 1 Drop Tests
Horse 1 was the lighter of the two horses, weighting 343 kg. The most repeatable data were
collected from the lumbosacral junction and sacral vertebrae impact locations (see Figure 5b,c). Both
left and right hind quarter locations were less reliable as the headform was pushed out of position in
some impacts (see Figure 5a,d). The highest mean peak loads applied to the headform were measured
at the sacral impact location (15.57 kN). The lowest mean peak loads were measured at the right hind
quarter (7.91 kN).
Figure 4.
(
a
) Left hind quarter impact location. (
b
) Sacral vertebrae impact location. (
c
) Lumbosacral
vertebrae impact location. (d) Right hind quarter impact location.
3. Results
Force–time data were successfully collected for all 30 drop tests with the split headform proving
to be reliable and repeatable.
3.1. Horse 1 Drop Tests
Horse 1 was the lighter of the two horses, weighting 343 kg. The most repeatable data were collected
from the lumbosacral junction and sacral vertebrae impact locations (see Figure 5b,c). Both left and right
hind quarter locations were less reliable as the headform was pushed out of position in some impacts (see
Figure 5a,d). The highest mean peak loads applied to the headform were measured at the sacral impact
location (15.57 kN). The lowest mean peak loads were measured at the right hind quarter (7.91 kN).
Table 1below summarises the peak force for each test, time to peak load for each test, means
and standard deviations for each impact location, and means and standard deviations for all impact
locations on horse 1. Time to peak load is presented, as this is within the dynamic loading phase of the

Appl. Sci. 2020,10, 2623 5 of 11
impact. At some stage the load transitions from being dynamic to essentially static and so it is difficult
to determine the full dynamic duration of the impact. The highest mean peak loads applied to the
headform by horse 1 were measured at the sacral impact location (15.57 kN
±
1.11 SD). The lowest
mean peak loads were measured at the right hind quarter (7.91 kN ±1.84 SD).
Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 12
Figure 5. Un-helmeted drop test force–time history plots for horse 1. (a) Left hind quarter impact
location. (b) Lumbosacral vertebrae junction impact location. (c) Sacral vertebrae impact location. (d)
Right hind quarter impact location.
Table 1 below summarises the peak force for each test, time to peak load for each test, means
and standard deviations for each impact location, and means and standard deviations for all impact
locations on horse 1. Time to peak load is presented, as this is within the dynamic loading phase of
the impact. At some stage the load transitions from being dynamic to essentially static and so it is
difficult to determine the full dynamic duration of the impact. The highest mean peak loads applied
to the headform by horse 1 were measured at the sacral impact location (15.57 kN ± 1.11 SD). The
lowest mean peak loads were measured at the right hind quarter (7.91 kN ± 1.84 SD).
Table 1. Un-helmeted drop test results for horse 1.
Test no.
Impact Location
Peak Force (kN)
Time to Peak (ms)
1
Left Hind Quarter
11.65
17.83
2
Left Hind Quarter
15.96
22.44
3
Left Hind Quarter
13.61
28
Mean
13.74
22.76
SD
2.16
5.09
1
Lumbosacral Junction
6.53
17.48
2
Lumbosacral Junction
8.28
70.04
3
Lumbosacral Junction
10.15
68.72
Mean
8.32
52.08
SD
1.81
29.97
1
Sacral Vertebrae
16.85
21.08
2
Sacral Vertebrae
14.89
25.16
3
Sacral Vertebrae
14.98
23.32
Mean
15.57
23.19
Figure 5.
Un-helmeted drop test force–time history plots for horse 1. (
a
) Left hind quarter impact location.
(
b
) Lumbosacral vertebrae junction impact location. (
c
) Sacral vertebrae impact location. (
d
) Right hind
quarter impact location.
Table 1. Un-helmeted drop test results for horse 1.
Test no. Impact Location Peak Force (kN) Time to Peak (ms)
1 Left Hind Quarter 11.65 17.83
2 Left Hind Quarter 15.96 22.44
3 Left Hind Quarter 13.61 28
Mean 13.74 22.76
SD 2.16 5.09
1 Lumbosacral Junction 6.53 17.48
2 Lumbosacral Junction 8.28 70.04
3 Lumbosacral Junction 10.15 68.72
Mean 8.32 52.08
SD 1.81 29.97
1 Sacral Vertebrae 16.85 21.08
2 Sacral Vertebrae 14.89 25.16
3 Sacral Vertebrae 14.98 23.32
Mean 15.57 23.19
SD 1.11 2.02
1 Right Hind Quarter 7.11 31.72
2 Right Hind Quarter 10.01 20.76
3 Right Hind Quarter 6.6 28.28
Mean 7.91 26.92
SD 1.84 5.61
Mean (All Locations) 11.39 31.24
SD (All Locations) 3.80 18.31

Appl. Sci. 2020,10, 2623 6 of 11
3.2. Horse 2 Drop Tests
Horse 2 was the heavier horse, weighting 370 kg. Again, the most repeatable data were collected
from the lumbosacral junction and sacral vertebrae impact locations (see Figure 6b,c). However, left
hind quarter data also showed good repeatability (see Figure 6a). The highest mean peak loads applied
to the headform were measured at the sacral impact location (16.02 kN). The lowest mean peak loads
were measured at the left hind quarter (10.47 kN). Overall, mean peak loads for all impact locations
were 1.52 kN greater (11.8%) for horse 2 impacts compared with horse 1 impacts.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 12
SD
1.11
2.02
1
Right Hind Quarter
7.11
31.72
2
Right Hind Quarter
10.01
20.76
3
Right Hind Quarter
6.6
28.28
Mean
7.91
26.92
SD
1.84
5.61
Mean (All Locations)
11.39
31.24
SD (All Locations)
3.80
18.31
3.2. Horse 2 Drop Tests
Horse 2 was the heavier horse, weighting 370 kg. Again, the most repeatable data were collected
from the lumbosacral junction and sacral vertebrae impact locations (see Figure 6b,c). However, left
hind quarter data also showed good repeatability (see Figure 6a). The highest mean peak loads
applied to the headform were measured at the sacral impact location (16.02 kN). The lowest mean
peak loads were measured at the left hind quarter (10.47 kN). Overall, mean peak loads for all impact
locations were 1.52 kN greater (11.8%) for horse 2 impacts compared with horse 1 impacts.
Figure 6. Un-helmeted drop test force–time history plots for horse 2. (a) Left hind quarter impact
location. (b) Lumbosacral junction vertebrae impact location. (c) Sacral vertebrae impact location. (d)
Right hind quarter impact location.
Table 2 below shows peak force for each test, time to peak load for each test, means and standard
deviations for each impact location, and means and standard deviations for all impact locations on
horse 2.
Figure 6.
Un-helmeted drop test force–time history plots for horse 2. (
a
) Left hind quarter impact
location. (
b
) Lumbosacral junction vertebrae impact location. (
c
) Sacral vertebrae impact location. (
d
)
Right hind quarter impact location.
Table 2below shows peak force for each test, time to peak load for each test, means and standard
deviations for each impact location, and means and standard deviations for all impact locations on
horse 2.
Table 2. Un-helmeted drop test results for horse 2.
Test no. Impact Location Peak Force (kN) Time to Peak (ms)
1 Left Hind Quarter 9.89 18.28
2 Left Hind Quarter 11.7 33.48
3 Left Hind Quarter 9.8 35.44
Mean 10.47 29.07
SD 1.08 9.39
1 Lumbosacral Junction 12.89 61.04
2 Lumbosacral Junction 13.64 64.6
3 Lumbosacral Junction 12.27 64.88
Mean 12.93 63.51
SD 0.69 2.14
1 Sacral Vertebrae 15.81 14.12
2 Sacral Vertebrae 16.94 40.36
3 Sacral Vertebrae 15.31 44.32
Mean 16.02 32.93
SD 0.83 16.41

Appl. Sci. 2020,10, 2623 7 of 11
Table 2. Cont.
Test no. Impact Location Peak Force (kN) Time to Peak (ms)
1 Right Hind Quarter 8.74 13.52
2 Right Hind Quarter 15.66 31.93
3 Right Hind Quarter 12.25 39.84
Mean 12.25 28.43
SD 3.46 13.5
Mean (All Locations) 12.91 38.48
SD (All Locations) 2.65 18.16
3.3. Horse 2 Helmeted Drop Tests
Figure 7shows that the helmeted tests were more repeatable in general when compared to the
un-helmeted tests, with good agreement between each test. As with the un-helmeted impacts, the sacral
impact location transferred the highest peak loads to the headform, with a mean of 11.26 kN, although
hind quarter locations were not tested in this case. Table 3below shows the peak force for each test,
the time to peak load for each test, as well as the means and standard deviations for each impact
location and means and standard deviations for all impact locations on horse 2 (helmeted tests).
Appl. Sci. 2019, 9, x FOR PEER REVIEW 8 of 12
Figure 7. Helmeted drop test force–time history plots for horse 2. (a) Lumbosacral junction vertebrae
impact location. (b) Sacral vertebrae impact location.
Table 3. Helmeted drop test results for horse 2.
Test no.
Impact Location
Peak Force (kN)
Time to Peak (ms)
1
Sacral Vertebrae
11.53
56.68
2
Sacral Vertebrae
10.05
54.6
3
Sacral Vertebrae
12.2
57.64
Mean
11.26
56.31
SD
1.1
1.55
1
Lumbosacral Junction
7.34
78.8
2
Lumbosacral Junction
6.66
71.56
3
Lumbosacral Junction
7.97
76.4
Mean
7.33
75.59
SD
0.66
3.69
Mean (All Locations)
9.29
65.95
SD (All Locations)
2.30
10.86
3.4. Helmeted vs. Un-helmeted vs. Impact Location
Tables 4 and 5 below show statistical comparison results of both un-helmeted and helmeted
impacts and pooled data comparisons for all impact locations.
Table 4. Helmeted vs. un-helmeted peak load comparisons for both combined and individual impact
locations. p-Values in bold indicate statistically significant differences. Note: These values are
calculated using un-helmeted data from horse 2 only as this was the horse used for helmeted impacts.
Helmeted vs. Un-Helmeted Impacts
p-Value
% Difference
Combined Locations
<0.001
43.60
Lumbosacral Junction
<0.001
55.40
Sacral Vertebrae
<0.05
36.73
Table 5. Pooled peak load comparisons for all impact locations. p-Values in bold indicate statistically
significant differences. LHQ and RHQ denote left and right hind quarters, respectively.
Impact Location Comparison
p-value
% Difference
Figure 7. Helmeted drop test force–time history plots for horse 2. (a) Lumbosacral junction vertebrae
impact location. (b) Sacral vertebrae impact location.
Table 3. Helmeted drop test results for horse 2.
Test no. Impact Location Peak Force (kN) Time to Peak (ms)
1 Sacral Vertebrae 11.53 56.68
2 Sacral Vertebrae 10.05 54.6
3 Sacral Vertebrae 12.2 57.64
Mean 11.26 56.31
SD 1.1 1.55
1 Lumbosacral Junction 7.34 78.8
2 Lumbosacral Junction 6.66 71.56
3 Lumbosacral Junction 7.97 76.4
Mean 7.33 75.59
SD 0.66 3.69
Mean (All Locations) 9.29 65.95
SD (All Locations) 2.30 10.86

Appl. Sci. 2020,10, 2623 8 of 11
3.4. Helmeted vs. Un-helmeted vs. Impact Location
Tables 4and 5below show statistical comparison results of both un-helmeted and helmeted
impacts and pooled data comparisons for all impact locations.
Table 4.
Helmeted vs. un-helmeted peak load comparisons for both combined and individual impact
locations. p-Values in bold indicate statistically significant differences. Note: These values are calculated
using un-helmeted data from horse 2 only as this was the horse used for helmeted impacts.
Helmeted vs. Un-Helmeted Impacts p-Value % Difference
Combined Locations <0.001 43.60
Lumbosacral Junction <0.001 55.40
Sacral Vertebrae <0.05 36.73
Table 5.
Pooled peak load comparisons for all impact locations. p-Values in bold indicate statistically
significant differences. LHQ and RHQ denote left and right hind quarters, respectively.
Impact Location Comparison p-Value % Difference
LHQ vs. RHQ >0.005 18.41
LHQ vs. Lumbosacral Junction >0.005 12.98
LHQ vs. Sacral Vertebrae <0.005 26.49
Lumbosacral Junction vs. Sacral Vertebrae <0.001 39.13
Lumbosacral Junction vs. RHQ >0.005 5.46
Sacral Vertebrae vs. RHQ <0.005 44.36
4. Discussion
4.1. Horse Impact Data
This study successfully collected quantitative force data from a horse falling onto a surrogate
headform. To the best of the authors’ knowledge, this is the first time that such data has been collected
and presented. In general, the repeatability of the force–time curves is good, despite movement of
the headform in some cases. Similarities in the shape of the loading curves can be seen for each
impact location and standard deviations about the mean values are small. Statistically significant
differences (p<0.05) in peak loads were observed between the left hind quarter and sacral vertebrae,
the lumbosacral junction and the sacral vertebrae, and between the sacral vertebrae and the right hind
quarter (see Table 5).
The sacral vertebrae impact location applied the highest peak loads to the headform in all cases.
This is likely due to the stiff, bony nature of this location with minimal soft tissue covering. Additionally,
the location is at the centre of the animal and, during the impact, there was minimal movement of the
headform, which ensured that it properly registered most of the associated load.
Quite the opposite was true of the hind quarter impact locations. There was much less headform
stability here as initial contact between the horse and the headform could cause the headform to be
pushed away from the horse. The lumbosacral junction vertebrae impact location proved to be very
stable and shows a very different loading curve when compared to the other impact locations. A steep
initial rise is followed by a much smaller slope, and in some cases a plateau. Due to the nature of
this location, this two-stage loading phase occurs when the initial contact between the horse and the
headform is first made and then, as the impact continues, the front and rear of the animal come into
contact with the ground, effectively reducing the effective mass applied to the headform. Additionally,
during each impact, the horse rotated as the legs came to rest on the concrete floor. It is possible that
the rotation of the animal caused some additional instability of the headform.
Horse mass appears to be a factor. A 7.3% increase in the mass of the falling horse resulted in,
on average, an 11.8% increase in the peak load applied to the headform. The horses used in this study
were not large and would typically fall into the pony category [
18
]. Much heavier animals are ridden

Appl. Sci. 2020,10, 2623 9 of 11
by equestrians and most sport horses weigh around 500–600 kg. A similar drop test with an animal of
such a larger mass would result in a significant increase in loads applied to the headform.
4.2. Helmet Effects
Helmeted drop tests were only carried out on the sacral and lumbosacral junction vertebrae
impact locations. Statistically significant differences (p<0.05) were observed between helmeted and
un-helmeted impacts for both impact locations (see Table 4). When compared with the un-helmeted
mean values for these locations, a reduction of 29.7% can be seen for the sacral impact location and
43.3% for the lumbosacral junction location. The helmet significantly reduces peak loads applied to the
headform. However, mean peak loads at the sacral and lumbosacral junction impact locations are still
high at 11.26 kN and 9.29 kN, respectively.
4.3. Skull Fracture
Force levels required for lateral skull fracture vary significantly. Mean values reported in the
literature range from 3.5 kN to 12.4 kN [
19
–
25
]. However, these forces vary with the surface area of the
impactor and the impact velocity [
20
]. It should also be noted that skull fracture occurs at much lower
loads for children [
26
]. Regardless, the measured loads in every test presented in this study fall within
or exceed this range. This includes the helmeted tests. Although the helmet can significantly reduce
the load applied to the head, it cannot protect against skull fractures in a severe scenario such as this.
However, this type of dynamic loading could be seen as an extreme case and it is quite possible that
the helmet would provide significant protection in cases where the horse merely rolls onto the head
rather than falls onto it. More research is required to determine if this is the case for such a quasi-static
loading scenario.
4.4. Limitations
The headform used was instrumented with a uniaxial load cell. This was chosen as a robust solution,
given the particular experimental conditions. However, a triaxial system would have provided more
information, particularly in cases where the headform moved during the impact. The headform itself
could be considered a rigid body and, therefore, would not respond as a human head would. However,
such a robust headform was required given the extreme nature of the tests carried out. The horses used
in this study may not have represented the heavier adult sport horses and thoroughbreds and, therefore,
might underestimate the actual risks occurring in equestrian sports. Nevertheless, this category of
a lighter horse potentially reflects the most vulnerable in equestrian sports, namely children riding ponies.
The impact locations were chosen because it was believed that they would ensure the most
repeatable results. It is not known if these locations commonly come into contact with a rider ’s head
during impact but, at present, this is unknown for any location. The study was limited by the number
of data points that could be collected and therefore, any statistical analyses were limited by the small
data set. More data are required to have confidence in statistical differences presented in this paper.
4.5. Future Work
More work is required to investigate loads applied to the head by horses of greater mass, different
impact locations, different drop heights and to understand what influence might be associated with
a saddle. Additionally, a more biofidelic headform that takes into consideration the scalp [
27
] and skull
may provide more insight. Such experiments are difficult and expensive to arrange. It is suggested
that the data presented in this present study could be used to validate a finite element or multibody
mathematical model, allowing alternative scenarios to be investigated.
The current standard lateral crush test method should be evaluated to see if there is any
relationship between this quasi-static test and dynamic crush. It may be possible to adapt current tests
as a cost-effective measure to ensure better helmet performance.

Appl. Sci. 2020,10, 2623 10 of 11
Current tests do not come close to simulating the loading conditions of real-world accidents in
a biofidelic manner and any future changes to a standard test method should have a firm evidence
basis to ensure the test is both useful and can lead to the desired safety outcome.
5. Conclusions
The data show that the force applied to the head from a falling horse can exceed the lateral skull
fracture tolerances, even if a helmet is used. The highest mean peak load applied to the headform
was 16.02 kN
±
0.83 SD. Peak loads were reduced by as much as 43.3% for helmeted tests. However,
all measured loads were within or exceeded the range of published data for the fracture of an adult
lateral skull bone. If the current standard lateral crush test continues to be used, some clarification
on the rationale for this test would be useful. However, if the equestrian and standards communities
believe that the lateral crush test used in certification standards is intended to provide some protection
against a horse falling onto a rider’s head, the data presented in this study should be taken into
consideration. It should be used as a basis for further experimental and numerical work on which to
develop new test methods, as no other data set yet exists.
Author Contributions:
Conceptualization, T.A.C., P.B., A.N.A. and M.D.G.; methodology and validation, T.A.C.,
J.M.C., P.B. and M.S.; analysis, T.A.C., J.M.C., A.N.A. and M.D.G., resources, project administration and funding
acquisition, P.B., M.S., A.N.A. and M.D.G.; writing—original draft preparation, T.A.C.; writing—review and
editing, all authors. All authors have read and agreed to the published version of the manuscript.
Funding:
This project received funding from the European Union’s Horizon 2020 research and innovation
programme under the Marie Sklodowska-Curie grant agreement No. 642662. The funding body had no role in the
design of this study or in the collection, analysis or interpretation of the data.
Conflicts of Interest: The authors declare no conflict of interest.
Declaration of Interest Statement:
No benefits in any form have been or will be received from a commercial
party related directly or indirectly to the subject of this manuscript.
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